Iron oxide based catalysts are used in the dehydrogenation of an alkylaromatic compound to yield, as the desired, main product, the corresponding alkenylaromatic compound.
However, when using the iron oxide based catalysts in the dehydrogenation of alkylaromatic compounds, several side reactions occur which decrease the yield of alkenylaromatic compound and therefore effect the economy of the process unfavorably. One such side reaction is the formation of coke on the catalyst, which reduces the lifetime of the catalyst. Other side reactions involve the formation of an alkynylaromatic compound, a methylaromatic compound, and a de-alkylated aromatic compound. For example, in the dehydrogenation of ethylbenzene, the desired, main product is styrene and undesired byproducts are coke, phenylacetylene, toluene and benzene.
In view of the applicability and use of the alkenylaromatic compound, the alkynylaromatic compound is frequently at least partly removed from the product of the dehydrogenation. This removal generally requires a separate process step, typically involving hydrogenation to the alkenylaromatic compound, using a selective hydrogenation catalyst.
U.S. Pat. No. 5,190,906 and EP-A-1027928 disclose that the source of the iron oxide for use in the iron oxide catalysts may be a process which comprises heat decomposition of an iron halide. By nature, such iron oxide catalysts produced by heat decomposition of an iron halide contain residual iron halide.
For example, dehydrogenation catalysts are in commercial use for the dehydrogenation of an alkylaromatic compound, which catalysts are based on an iron oxide made by heat decomposition of an iron halide, and further contain a small amount of a lanthanide, for example 0.066 mole per mole of iron oxide, calculated as Fe2O3.
When in the dehydrogenation of an alkylaromatic compound such commercially available catalysts are applied alkynylaromatic compound is co-produced. It would be highly desirable to decrease the selectivity to the co-produced alkynylaromatic compound. As used herein, the selectivity to a particular compound means the fraction of the converted alkylaromatic compound yielding the particular compound.